The subject matter of this disclosure is generally related to techniques and tools used in well logging, particularly in oil and natural gas exploration and production. Well logging tools probe geologic formations using probe signals, and determine characteristics of the formations based on interaction between the probe signals and the formations. More specifically, the present technologies relate to amplifying the probe signals and to reducing distortion in the amplified signals.
Well logging refers to the practice of recording information about geological formations penetrated by a wellbore and/or about fluids found within the wellbore. Wireline logging refers to well logging that is performed by lowering a well logging tool on the end of a wireline into the wellbore and recording petrophysical properties using a variety of sensors. Well logging can be performed after the wellbore has been completed or during the drilling process (referred to as “logging while drilling”).
Information obtained from wireline logging may be used for a variety of purposes including, for example, deciding whether to continue at a current drilling location or to choose a subsequent drilling location, or to determine the location of a drill bit (e.g., a depth under the earth surface) and the direction that drilling should follow.
Logging tools can be used to probe a variety of characteristics of the geological formations penetrated by a well. A logging tool can be a resistivity tool, a density tool, a sonic or acoustic tool, or an imaging tool respectively operated with audio frequency, gamma, sound or radar frequency signals. The information generated by the logging tool about a geological formation is derived through the interaction of probe signals with that formation. Typically, a probe signal is launched by an excitation source of the logging tool into a wellbore fluid, a geological formation or both, and a receiver of the logging tool is used to measure a characteristic of the probe signal such as, for example, an amount of time required by the probe signal to travel a given distance into the fluid and/or formation, or an attenuation of the probe signal over the traveled distance.
When a linear amplifier of a logging tool is operated in a linear amplification range, an amplified signal is a direct, linear function of the probe signal that is input to the linear amplifier: amplified signal=(amplifier gain)×(input probe signal). Linearity refers to the ability of the amplifier to produce amplified signals that are accurate copies of the input probe signal, generally at increased powers. Neither clipping nor distortion (harmonic distortion or intermodulation distortion) occurs when the linear amplifier is operated in the linear amplification range (although random noise may still be introduced).
Like reference symbols in the various drawings indicate like elements.